1. Technical Field
The present invention relates to a signal processing circuit.
2. Description of the Related Art
The FM airwaves have the same nature as light and, therefore, are reflected by buildings and mountains. Since reflected wave is temporally delayed and arrives at a reception antenna after direct wave, the reflected wave may interfere with the direct wave. If the reflected wave is strong, feeling of noise and distortion may be generated in sound output from a receiver and a degree in separation may be reduced in the case of stereo broadcast. Such a reception difficulty due to the reflected wave is referred to as multipath jamming and the generated noise is referred to as multipath noise.
An FM receiver, especially, a non-vehicle FM receiver is easily affected by the multipath noise since a relationship between the direct wave and the indirect wave changes momentarily as a vehicle moves.
Therefore, some FM receivers include a signal processing circuit that detects generation of multipath noise to attenuate a stereo composite signal demodulated by FM detection during a period when the multipath noise is detected (see, e.g., patent document 1). A conventional signal processing circuit detects a period of generation of the multipath noise based on a change in the stereo composite signal intensity and a change in an AC component of a signal meter and attenuates the stereo composite signal to reduce the feeling of noise in accordance with an attenuation rate obtained from the stereo composite signal intensity during a period when the generation of the multipath noise is detected.
Patent document 1: Japanese Patent Application Laid-Open Publication No. 2003-283349
In the conventional signal processing circuits, the attenuation rate of the stereo composite signal at the time of noise detection is set only from the stereo composite signal as described above. However, the optimum value of the attenuation rate is varied by electric field intensity and reception states such as a level of a noise component included in the received signal. Therefore, the conventional signal processing circuits cannot set the attenuation rate appropriately and the feeling of noise and distortion remains. Since the conventional signal processing circuits detects the multipath noise based on the AC component of the signal meter, if the spectrum the AC component is disturbed, the detection accuracy of the multipath noise is deteriorated. If the attenuation rate is high and the generation period of the multipath noise is long, a sound disconnection phenomenon is generated and output sound is discontinued.
The object of the present invention is to provide a signal processing circuit that can set the attenuation rate in consideration of the change in the envelope curve obtained based on the intermediate frequency and the electric field intensity in addition to intensity of a stereo composite signal to reduce the feeling of noise and distortion due to multipath noise.